hi,
first make sure, that your CGI programs start like this:

use strict;
use warnings;
use CGI;
[download]

this will save you later from a lot of pain :). You can find more information on this on the Tutorial section here on Perlmonks: use strict and warnings , use CGI or die; .
Some good information on session management you will find reading CGI::Session::Tutorial .

After some time, you will realise that there is a more better way of writing CGI programs: using CGI::Application . It is a "Framework for building reusable web-applications". It has great plugins too (Sessions, Authentication/Authorization, Form Input Validation, etc.).
Some tutorials/examples about using CGI::Application:

• Authentication with CGI::Application
• An Example of a login/logout application with CGI:::Application
• CGI::Application::Plugin::Session Example

I hope this is usefull for you. Altough i'm programming Perl and reading Perlmonks for over 1 year, this is my first post. I learned a lot from the monks. Thank you!